1. Field of the Invention
The present invention relates to a control system of a vehicle having a continuously variable transmission capable of changing a gear ratio continuously and, more particularly, to a control system for controlling both a power source and a gear ratio in response to a demand for a driving force.
2. Related Art
In recent years, a continuously variable transmission has been adopted as a transmission for a vehicle. The continuously variable transmission of this kind is so constructed that the gear ratio is continuously changed by changing the groove width of an input side pulley and an output side pulley, on which a belt is made to run, that is, a belt winding radius, or that the gear ratio is continuously changed by inclining a power roller interposed between an input side disc and an output side disc, each of which has a troydal face, to change the radius of the position for the power roller to contact with the input side disc and the output side disc. Moreover, the gear ratio is controlled on the basis of the running state of the vehicle, which is decided, for example, from a depression angle (or accelerator opening) signal of an accelerator pedal, as indicating the output demand of a driver, an output demand signal coming from a cruise control for keeping the vehicle speed at a constant speed, and the vehicle speed.
According to the continuously variable transmission, the gear ratio can be continuously changed, and the output of the power source such as an engine is continuously changed. In Japanese Patent Publication No. 72867/1991 (JPB3-72867), therefore, there is disclosed a control system which is constructed to perform a run of excellent efficiency by making use of the characteristics of the continuously variable transmission effectively. In the disclosed system, the target driving force is determined from the accelerator pedal depression and the vehicle speed to determine the target engine output corresponding to the target driving force, the target throttle opening and the target engine speed, and the target gear ratio is determined on the basis of the target engine speed and the vehicle speed. In the disclosed system, moreover, the target engine output and the target gear ratio are determined on the basis of an optimum fuel economy curve.
According to the control system of the prior art thus far described, therefore, when the accelerator pedal is depressed, for example, the target driving force and the target vehicle speed are determined on the basis of the accelerator pedal depression and the vehicle speed, and the change from the running state at that time to the target running state occurs along the best fuel economy curve. In the system of the prior art, therefore, even when an acceleration is demanded by depressing the accelerator pedal, the controls to increase the fuel supply temporarily and to raise the engine speed temporarily are not made, but what is controlled is to optimize (or minimize) the fuel consumption. As a result, the engine torque or the driving torque fails to satisfy the demand for acceleration of the driver, and the vehicle may have an inferior power performance and a poor drivability.